January 2017 Bulletin

Magnetic Basement Structures

Northwestern East Texas Basin

Michael Alexander

Geophysical Advisor, Integrated Geophysics Corp.

Introduction

The deep onshore northern Gulf Coast Basin has been intensely explored, both regionally and in detail, using gravity and reflection seismic geophysical surveys for well over 50 years. Although both survey methods allowed successful mapping of structures associated with salt and/or other sediments, neither method provided significant detailed information about “crystalline” basement structure and the primary igneous basement architecture beneath the sedimentary section and the deeper metamorphics. This paper concentrates on basement trends along the northwestern corner of the East Texas Basin, under and along the Ouachita-Mexia-Talco fault zones. Neither the sediments nor the metamorphics are known to be conformable with crystalline basement structure along those zones.

Seismic mapping of crystalline basement on either a regional or a local basis was not feasible until field techniques and data processing were improved to the point of allowing reasonable interpretation of deep metamorphic or pre-metamorphic features. However, “reasonable” does not imply “unique” depth interpretation because there are velocity and reflector identification uncertainties. Some non-uniqueness of the reflection seismic interpretation can be reduced by constraining it with quantitative magnetic depth analysis, integrated 2D gravity/magnetic modeling, plus any available seismic refraction data.

The Precambrian and Paleozoic sediments in this northwestern East Texas area were involved in the Ouachita orogeny that produced regional compression in a belt that spans from southwest Texas through southern Oklahoma. The north-northeast trend of the Ouachita Tectonic Belt follows the Mexia Fault Zone into Hopkins County, Texas. Then the Mexia changes direction and bends abruptly eastward to become the Talco Fault Zone. The Talco continues easterly toward southwestern Arkansas. To the south, the Mexia is flanked by strong magnetic maxima representing basement high blocks which are offset along trend by northwest-southeast linear features.

Figure 1 is an index map of the area showing county lines, regional structures, key wells, and the locations of three 2D gravity/magnetic structural models.

Figure 2 is a total magnetic intensity contour map of the area. The magnetic data are from USGS digital files with shaded contour values in nanoteslas.

Figure 3 is a Bouguer map of central Texas. The gravity data are from USGS digital files and have been mapped by University of Wisconsin - Green Bay.